1. Field of the Invention
The present invention relates to a marking ink composition comprising a dye and a pigment, and a display medium comprising the above-mentioned marking ink composition which is held in an enclosed space. Further, the present invention also relates to a printing method using the above-mentioned marking ink composition, and an image printed on an image receiving material by such a printing method.
2. Discussion of Background
A liquid crystal, electrochromic device, electrophoretic migration device, and magnetic migration device are conventionally known as elements for use in the display medium which can be visually recognized as reversibly changeable by the action of an electric field. Most of such display media employing the above-mentioned elements are constructed in such a manner that the display element is held between a pair of electrodes, with the display medium being connected to a driving circuit capable of applying a signal to each electrode for achieving image display.
The conventional display medium of electrophoretic migration type employs a marking ink composition which comprises a liquid medium, a dye which is dissolved therein, and a pigment powder which is dispersed therein. A marking ink in which two kinds of pigment powders are dispersed is also conventionally used. The display media using such marking ink compositions are described in Japanese Patent Publication 50-1519 and U.S. Pat. No. 3,668,106.
However, it is difficult to use the above-mentioned marking ink composition in which a dye is soluble as a printing ink for forming an image on a sheet of paper. When such a marking ink is applied to a display medium, the contrast and the reflectance are inferior to those of an image formed by an electrophotographic process, as reported in D. W. Vace, Proc. SID, vol.18/3and4, p.267, 1977, and J. Jacobson, Nature, vol.394/16, p.252, 1998. This is because inherent color of the pigment powder is hindered by a dye adsorbed by the surface of the pigment powder and a dye solution existing between the pigment particles, and therefore the color of the pigment is unfavorably mixed with the color of the dye.
On the other hand, when the above-mentioned conventional marking ink composition in which two kinds of pigment powders are dispersed is employed, although the contrast can be easily increased, the response speed is very slow in electrophoretic migration. The reason for this is that the two kinds of pigment powders which are provided with different electric charges are lacking in dispersion stability. Further, there is the drawback that aggregation of the two kinds of pigment powders results in color mixing, thereby decreasing the contrast.
In addition, a display medium using a liquid which comprises polarized particles, which medium is similar to the above-mentioned display medium of electrophoretic migration type, is proposed in Japanese Patent Publication Nos. 54-15217 and 57-25811. However, such a display medium has the problems in long-term stability, response speed, and display uniformity although the contrast is relatively high.
There is also conventionally known a magnetic migration type display medium. The state of the display medium can be reversibly switched by the action of a magnetic field, and such states can be visually recognized. One of the magnetic migration type display media comprises a marking ink composition in which a magnetic powder and a pigment powder serving as a concealing powder are dispersed in a liquid medium. This ink composition is held in an enclosed space. The migration of the magnetic powder in the liquid medium is caused by the application of a magnetic field to the liquid medium using a magnet, so that reversible color display can be achieved. The magnetic migration type display medium, which is described in, for example, Japanese Patent Publication Nos. 51-10959 and 57-27463, is utilized in the fields of toys for children and blackboards.
Similar to the above-mentioned electrophoretic migration type display medium using the two kinds of pigment powders, the display medium of magnetic migration type has the drawback that the two kinds of powders tend to aggregate to produce the problem of color mixing, so that the contrast is easily decreased.
To eliminate the above-mentioned drawback, a display medium using a magnetic fluid instead of the magnetic powder is proposed in Japanese Laid-Open Patent Application No. 51-93827. In this case, however, it is pointed out that the decrease of contrast is inevitable after long-term service because of unstability of the magnetic fluid.
To improve the display medium using the above-mentioned magnetic powder or magnetic fluid, Japanese Laid-Open Patent Application 10-116038 discloses a marking ink composition comprising two different kinds of dispersion media for forming two immiscible liquid phases, and pigment powders with two different colors which are separately dispersed in the respective dispersion media. This ink composition is intended to improve the contrast by completely separating a coloring powder from a concealing powder. Therefore, the pigment powders with different colors are separately dispersed in the respective dispersion media. The aforementioned marking ink composition is characterized in that the two dispersion media are capable of forming two layers which are immiscible, and that each pigment powder shows a strong affinity for the dispersion medium in which the pigment powder is dispersed.
However, there is a problem in this type of display medium. Namely, two different colors are reversibly displayed by using the two kinds of pigment powders, so that there often occurs physical collision between the different kinds of pigment powders while the position of the pigment powders are replaced for switching the color display. Such physical collision causes the two kinds of pigment powders to aggregate, thereby decreasing the response speed. At the same time, the contrast is decreased because of color mixing. In particular, when the two kinds of pigment powders are electrophoretically driven, the characteristics of those pigment powders are caused to deteriorate conspicuously by the collision. Furthermore, the concentration of pigment powder is increased with the progress of the aggregation of the two kinds of pigment powders, thereby increasing the viscosity of the liquid medium as the dispersion. The result is also the decrease in response speed.
In light of the above-mentioned conventional shortcomings, a first object of the present invention is to provide a marking ink composition capable of reversibly displaying at least two colors with excellent contrast or high degree of whiteness.
A second object of the present invention is to provide a display medium capable of reversibly displaying at least two colors using the above-mentioned marking ink composition.
A third object of the present invention is to provide a printing method using the above-mentioned ink composition.
A fourth object of the present invention is to provide an image printed by the above-mentioned printing method using the ink composition.
The first object of the present invention can be achieved by a marking ink composition comprising a dye, a pigment, a dispersion medium X in which the dye is practically soluble, and a dispersion medium Y in which the dye is practically insoluble, the dispersion medium X and the dispersion medium Y being mutually immiscible.
The second object of the present invention can be achieved by a display medium comprising the above-mentioned marking ink composition, and two plates for holding the marking ink composition therebetween, at least one of the plates comprising a transparent portion.
The third object of the present invention can be achieved by a method for printing an image on an image receiving material, comprising the steps of depositing the above-mentioned marking ink composition on the image receiving material, and removing the dispersion medium X and the dispersion medium Y from the image receiving material.
In the above-mentioned depositing step, the dispersion medium X in which the dye is soluble may be first deposited on the image receiving material to form a first image thereon, the dispersion medium Y in which the pigment is dispersed may be superimposed on the first image.
The fourth object of the present invention can be achieved by an image printed on an image receiving material by the above-mentioned printing method.